Microbubble generator for the transfer of oxygen to microbial inocula and microbubble generator immobilized cell reactor

ABSTRACT

A microbubble generator is disclosed for optimizing the rate and amount of oxygen transfer to microbial inocula or biocatalysts in bioreactor systems. The microbubble generator, and an associated immobilized cell reactor, are useful in the detoxification and cleanup of non-volatile polymeric and volatile organic-contaminated aqueous streams. In particular, they are useful in the continuous mineralization and biodegradation of toxic organic compounds, including volatile organic compounds, associated with industrial and municipal effluents, emissions, and ground water and other aqueous discharges. One embodiment of the invention includes a microbubble chamber packed with small inert particles through which a liquid effluent and oxygen or another gas are admitted under pressure, followed by a venturi chamber to further reduce the size of bubbles.

FIELD OF THE INVENTION

This invention relates to a microbubble generator for use in optimizingthe rate and amount of oxygen transfer to microbial inocula orbiocatalysts in bioreactor systems. In particular, it relates to amicrobubble generator and immobilized cell reactor as an apparatuscombination useful for the detoxification and cleanup of non-volatilepolymeric and volatile organics-contaminated aqueous streams, preferablythe latter.

BACKGROUND

Organic pollutants from diverse agricultural, municipal and industrialfacilities, and waste sites partition preferentially into the soil,water or air phases and spread rapidly throughout the environment. Manyof these materials, even in small concentrations, adversely affect lifeforms, and create serious environmental threats. Toxicity,carcinogenicity, and mutagenicity are the most critical biologicalproperties of a potential pollutant, a very large number of organics ofwhich have been identified by the U.S. Environmental Protection Agencyas particularly threatening. For example, the widespread use of the lastfew decades of herbicides, pesticides and related high risk chemicals,e.g., organochlorines, polychlorobiphenyls (PCB's) and chlorinatedphenols, have resulted in serious environmental problems.

In application Ser. No. 519,793, filed May 7, 1990, by David D. Fridayand Ralph J. Portier, now abandoned there is disclosed a continuousflow, immobilized cell reactor, and bioprocess, for the detoxificationand degradation of volatile toxic organic compounds. The reactor isclosed, and provided with biocatalysts constituted of specific adaptedmicrobial strains immobilized and attached to an inert porous packing,or carrier. A contaminated groundwater, industrial or municipal waste,which is to be treated is diluted sufficiently to achieve biologicallyacceptable toxicant concentrations, nutrients are added, and the pH andtemperature are adjusted. The contaminated liquid is introduced as aninfluent to the closed reactor which is partitioned into two sections,or compartments. Air is sparged into the influent to the firstcompartment to mix with and oxygenate the influent with minimalstripping out of the toxic organic compounds. The second section, orcompartment, is packed with the biocatalyst. The oxygenated liquidinfluent is passed through the second compartment substantially in plugflow, the biocatalyst biodegrading and chemically changing the toxiccomponent, thereby detoxifying the influent. Non toxic gases, and excessair from the first compartment, if any, are removed through a condenserlocated in the overhead of the reactor. Liquids are recondensed back tothe aqueous phase via the condenser.

In the reactor described in the application, supra, the air in the formof a high velocity jet is sparged into the first compartment of thereactor and combined with the conditioned liquid influent at highgas/liquid shear conditions, under pressure, to create very finebubbles. The mixed phases of air and liquid are flowed through a bedpacked with a solid inert packing, e.g., glass beads, then through anopen tubular column to a packed bed of biocatalyst. Good mixing, withminimum stripping of the toxic organic compounds from the liquid, isobtained. The pressure increases the oxygen driving force, and the highgas shear provided by the method of contact between the gas and liquid,and solid inert packing, under pressure, minimizes bubble diameter andincreases the interfacial transfer area between the phases. By smallbubble formation and good mixing with good air/liquid contact atelevated pressure, the volume of air that is introduced into the reactoris minimized. Consequently, a lesser amount of the volatile organiccompounds are stripped from the liquid and a greater concentration ofthe volatile organic compounds are contracted, with the biocatalyst andmineralized to detoxify the influent stream. Whereas the apparatusdescribed in this application has proved admirably suitable for admixingthe liquid and oxygen influent phases introduced into the reactor, withminimal stripping of the volatile organics by the gas, there nonethelessremains need for alternate bubble generation devices, as well as a needfor improved bubble generation devices. There also remains a need forreactors of improved design for carrying out these types of reactionsdue principally to the volatility of many of the chemical toxicantstargeted for detoxification biodegration.

OBJECTS

It is, accordingly, a primary objective of this invention to satisfythese and other needs.

In particular, it is an object to provide a novel microbubble generatorfor achieving high transfer rates and dissolved oxygen levels with airor other oxygen-containing gas, under pressure, by generating bubbles ofdiameter approaching the cell diameter of a great number, if not most ofthe known microorganisms.

A more specific object is to provide a microbubble generator, ascharacterized, and immobilized cell reactor combination useful for thecontinuous mineralization and biodegradation of toxic organic compounds,notably volatile organic compounds associated with industrial andmunicipal effluents, emissions, ground water and other aqueousdischarges, especially at conditions which provide optimum ornear-optimum detoxification of said toxic streams.

THE INVENTION

These objects and others are achieved in accordance with the presentinvention which embodies a microbubble generator useful in facilitating,improving, or achieving high oxygen transfer rates and dissolved oxygenlevels in microbiologically-mediated processes, or processes wherein thetransfer of the oxygen to microbial inocula, or biocatalysts, isrequired.

The apparatus, in one of its aspects, is comprised generally of aninitial, or first chamber, generally termed a microbubble chamber,packed with spherical inert solids particles of small diameter, inletsthrough which oxygen or an oxygen-containing gas, suitably air, and aliquid influent can be admitted under pressure into the chamber andcontacted together to form a mixed liquid-gas phase, or liquid stream inwhich the gas is dispersed as small bubbles, and an outlet through whichthe mixed liquids-gas phase is removed from the chamber. The apparatusfurther includes a second chamber or tubular section, of venturiconfiguration provided with an inlet through which the mixed liquid-gasphases are forced under pressure to further reduce the size of thebubbles of the mixed liquid-gas phases, and an outlet for discharge ofthe mixed liquid-gas phases.

In its more preferred form, the microbubble generator is constitutedgenerally of a dual compartmented vessel, a chamber of generally venturiconfiguration surrounded by an enclosing outer compartment, or chamber.The microbubble, or outer chamber is filled with non-reactive or inertspherical particles, e.g., glass beads, and provided preferably at oneof its ends, or sides, with inlets through which an influent liquid andan oxygen-containing gas can be introduced into said microbubblechamber, and admixed. An outlet at the opposite end, or side of thechamber constitutes an inlet to the chamber of venturi configuration.The mixed liquid-gas phases are introduced into this inlet to the secondchamber, or chamber of venturi configuration, and forced under pressurethrough the constructed cross-section thereof to further reduce the sizeof the bubbles of the mixed liquid-gas phases. The mixed liquid-gasphases are then discharged via an outlet from the second, orventuri-shaped chamber.

The microbubble generator is found particularly useful in combinationwith bioreactor systems to increase the oxygen transfer rates anddissolved oxygen levels in microbiologically-mediated reactions, ase.g., in the mineralization of polymeric or complex carbon substrates inwaste waters where high oxygen demand is necessary, or biodegradation ofvolatile toxic organic compounds as present in contaminated ground waterand waste water from industrial, municipal or other sources. In thisform, an oxygenated liquid from the microbubble generator is thusintroduced as an influent with compressed air into a biological reactor,suitably an immobilized cell reactor, wherein a continual oxygentransfer to a microbial mass is facilitated by the small bubble diameterand large number of bubbles which increases the rate, or quantity of themetabolic end products from the reactor. A microbubble generator of thistype allows super saturated dissolved oxygen levels to be achieved in abioreactor system by generating a large number of bubbles approachingthe cell diameter of most microorganisms, generally of average celldiameter ranging from about 0.10 millimeters (mm) to about 3 mm, or evenfrom about 0.25 mm to about 1 mm, which is preferred. At these smallbubble diameters theoretical transfer rates of about 16% to 18% arereadily achievable in virtually any fluidized bed, packed bed, oractivated sludge reactor system. The bed life of a reactor, it is found,is promoted by an optimal or near-optimal transfer of oxygen to thebiomass. Kinetic removal rates are optimized as a result of the improvedoxygen transfer, this providing reduced compressed air capacity withhigh degradation of the treated material, and minimal attrition ordamaging of the carrier bed during the operation. Since bed life ispromoted, significant economic advantages are realized in theestablishment of a biocatalytic unit having an operational lifegenerally measured in years of operation rather than in weeks or months.

The microbubble generator, in a preferred combination is thus employedas the first stage of a bioreactor unit. The microbubble generator isused to oxygenate under pressure a liquid stream containing e.g., atoxic volatile organic compound, diluted as necessary with water andprovided with nutrients to optimize the concentration of the toxicorganic compound and nutrient concentration, to a bioreactor formicrobial attack by selected microorganisms having a high metabolicuptake rate. In an initial step, foreign solids particulates may beremoved, if necessary, from the contaminated raw aqueous liquid, e.g.,via filtering. The liquid, from which particulate solids have beenremoved, can then be diluted with water to the degree necessary toprovide biologically acceptable toxicant concentrations for theimmobilized microorganisms of the catalyst selected to carry out themicrobial attack. Nutrients (e.g., K, P, N) such as are needed toprovide the energy and growth of the selected microorganisms are added,and the temperature of the liquid is adjusted. An acid, e.g., a mineralacid such as HCl, or a base, e.g., NaOH, is added as may be required toadjust the pH of the liquid to that at which the reaction is to beconducted. In general, the temperature of the contaminated liquid fedinto the reactor is adjusted to, and maintained within, a range fromabout 25° C. to about 35° C., preferably from about 28° C. to about 32°C., and pH ranging from about 6 to about 8, preferably from about 6.8 toabout 7.2, dependent largely on the specific nature of themicroorganisms used to form the biocatalyst employed in the packed bedportion of the reactor. The reactor into which the conditioned influentis fed is operated in a closed flow system under pressure, suitably atpressure ranging from about 15 pounds per square inch gauge (psig) toabout 50 psig, preferably from about 30 psig to about 40 psig, dependentalso on the specific nature of the organism.

A preferred microbubble generator, microbubble generator-reactorcombination, and the principles of operation of both will be more fullyunderstood by reference to the following detailed description, and tothe drawing to which reference is made in the description. The variousfeatures and components in the drawing are referred to by numbers,similar features and components being represented in the different viewsby similar numbers. Subscripts are used in some instances with numberswhere these are duplicate parts or components, or to designate asub-feature or component of a larger assembly.

REFERENCE TO THE DRAWING

In the drawing:

FIG. 1 depicts a sectional side elevation view of the microbubblegenerator.

FIG. 2 depicts the combination of the microbubble generator andcross-section of the reactor.

FIG. 3 is a graphical illustration of the data exemplified hereafter.

The combination is constituted generally of a microbubble generator andan instrumented bioconversion reactor. The unit as a whole furtherincludes storage reservoirs for pH adjustment, interconnecting pipingand valve manifold, pumps, thermostats and flow control regulationdevices, and a free-standing control panel which houses processmonitoring devices and signal conditioning electronics. Forsimplification most of these components of the unit other than themicrobubble generator and reactor are not shown, or are illustratedschematically.

Referring to FIG. 1, the microbubble generator is constituted of a dualcompartmented vessel 10, a first compartment, or chamber 9 defined bythe space within the enclosing side wall 11, top wall 12 and bottom wall13, and second compartment, or chamber 8, defined by the space insidethe enclosing wall 14 which defines generally a vertically orientedtubular member of venturi configuration. The tubular member 14 isvertically supported, essentially concentrically within the confines ofthe enclosing side wall 11 of the vessel at the top wall 12, throughwhich the member is projected and attached, and at its bottom end via adrain tube 15 which is projected through the bottom wall 13 of thevessel, to which it is attached. An outlet 21 provides a means for theremoval of gases and liquids from the chamber 8 of tubular member 14.Web connections provide a means for attachment of the bottom end ofdrain tube 15 to the tubular member 14, while at the same time leavingan essentially annular passageway, or inlet 16, for communicationbetween chambers 8, 9. The drain tube 15 during normal operation isclosed. It is opened for draining off excess of by-product liquid duringshut down periods. A liquid influent is introduced into the outer, orfirst compartment, or chamber 9, via inlets 17, 18, and anoxygen-containing gas, suitably air, via inlets 19, 20, which areconcentrically mounted within the liquid inlets 17, 18; all of which arelocated in the top wall 12 of the enclosing wall 11 of the microbubblegenerator 10. A gas introduced under pressure via inlets 19, 20 into thevery center of the axis of flow of the liquid, introduced via inlets 17,18 immediately begins to form bubbles within the entering liquid.

The inside face of the enclosing side wall 11 is provided with a seriesof two or more spaced apart, generally parallel, concentrically mountedbaffles 22₁, 22₂ which direct the mixed phase flow of liquid and gastoward the axis, or center of the vessel 10. Baffles 23₁, 23₂ of similarconstruction and orientation are mounted on the inside face of wall 14,these also directing the mixed phase flow of liquid and gas toward theaxis, or center of tubular member 14 as these phases are directed towardand forced through the restricted cross-sectional area, or orificeportion of the tube. The chamber 9 in operation is filled with smalldiameter spherical solids particles 24 which are inert, or non-reactivewith the processed materials, these shearing the gas and aiding theformation of a large number of bubbles of small diameter within thecompartment, or chamber 9; and the formation of a larger number ofbubbles of even smaller diameter within the chamber 8 when the mixedliquid-gas phase stream is forced therethrough. An oxygen-containinggas, preferably air, combined with the liquid at these conditionscreates very fine gas bubbles, generally bubbles of average diameterranging from about 0.10 to about 3 mm, and most often from about 0.25 mmto about 1 mm, sufficient to obtain good mixing and oxygen masstransfer. An opening, not shown, which is closed, covered or cappedduring normal operation can be provided within the enclosing side wall11 for charging, or removing, the inert solids particles 24; or thisfunction can be provided by removal of the top wall 12 which constitutesa cover or capping member for the vessel 12.

Referring to FIG. 2, the bioconversion reactor 30 is constitutedgenerally of a vertically oriented vessel of elongate, or tubular shape,formed by the enclosing side wall 32, top cover 27 and bottom cover 28.The reactor 30 is packed with a bed 31 of particulate biocatalystcomposition supported upon the porous distribution medium, perforatedplate or frit 29 located at the lower end of the enclosing side wall 32.An inlet 33, located in the bottom cover 28 of the reactor 30, providesa means for the continuous introduction of a stream of the mixedliquid-gas phases from the microbubble generator 10 via line 21 into thereactor. The oxygenated liquid stream, on entering the reactor, isflowed continuously upwardly through the biocatalyst bed 31, and isremoved from the reactor via overflow through line 35. A portion of thetreated stream may be recycled with the influent of fresh feed toincrease the fluid retention time and control the extent of toxiccompound conversion within the reactor. Suitably, the retention time ofthe liquid within the reactor ranges from about 8 hours to about 36hours, preferably from about 12 hours to about 16 hours. High recyclerates of the treated stream:fresh feed are preferred, the recycle rateof the treated stream:fresh feed ranging from about 10:1 to about 100:1,preferably from about 50:1 to about 75:1.

The reactor is pressurized by the gas sufficiently that on theintroduction into the reactor of an organics-containing, or gas orvapor-containing, aqueous liquid, as found e.g., in contaminated groundwater or in an industrial or municipal stream, the organics or gas andvapor will be maintained in or solubilized back into the aqueous phase.The pressure also regulates and controls the amount and transfer rate ofdissolved oxygen to the immobilized biomass or biocatalyst bed, thispermitting optimal oxygen transfer to the entrenched biomass withminimal attrition or damaging of the carrier bed during operations. Bedlife is promoted, providing a biocatalytic unit having an operationallife measured in years rather than in months, or weeks as generallyoccurs in conventional processing. In general, the total pressuremaintained on the reactor during operation ranges from about 15 psig toabout 50 psig, preferably from about 30 psig to about 40 psig.

A liquid level control, not shown, maintains the level of the liquid inthe reactor, the water being removed from the reactor via line 35. Gasrises into the overhead condenser 36 of the reactor, condensate beingreturned to the liquid surface of the reactor on contact with the coppercoils, cooled via liquid coolant entering therein via line 37 andexiting therefrom via line 38. Gas passes out of the overhead of thereactor via line 39.

The microorganism immobilized and attached to the surface of thecatalyst support is selected, with the conditions of operation imposedthereon, to drive the reaction to produce maximum breakdown of the toxicorganic compound, or compounds, to non-toxic components. The liquid inthe reactor is maintained and controlled by a level controller, notshown, at a level above the upper surface of the bed; liquid influentbeing continuously added to the bottom of the reactor via line 21, andwithdrawn from the upper portion of the reactor via line 35. Liquideffluent withdrawn from the reactor via line 35, for recycle, isreconditioned by readjustment of the dilution factor, nutrientconcentration, temperature and pH, and then oxygenated and recycled tothe reactor.

Streams containing a wide variety of toxic organic compounds can bedetoxified pursuant to the practice of this invention, particularlycompounds having Henrys' Law Constants ranging from 0.025 dimension lessmole fraction, and greater. These include aliphatic and aromatichydrocarbons, and the halogenated derivatives thereof a large number ofwhich are listed on the EPA List of Priority Pollutants. [Keith L. H.and Telliard W. A., Priority Pollutants, a Perspective View,Environmental Science and Technology, Vol. 13, pp. 416-423 (1979).]

With the preferred biocatalysts the half-lives of most toxic volatileorganic, or toxic volatile halogenated organic compounds will rarelyexceed one day, and most will range between about 4 hours and 16 hours.It has been found that such organic compounds as benzene, toluene, ethylbenzene and xylenes, or such halogenated organic compounds as, e.g.,1,1-dichloroethane, 1,2-dichloroethane, methyl chloroform,1,1,2,2-tetrachloroethane, hexachloroethane, bromoethane, and1,2-dibromoethane, respectively, contained in a conditioned liquid canbe 99.9%, by weight, converted to non-toxic products at a total reactorresidence time ranging below one day via the use of biocatalystsutilizing immobilized microorganisms of a select class which canmetabolize the organic or halogenated organic compounds as carbon andenergy sources, e.g., a strain of Xanthobacter autotrophicus.Xanthobacter autotrophicus is member of a known biologically uniquegroup of bacteria which have the ability to growchemolithoautotrophically in gas atmospheres containing hydrogen, oxygenand carbon dioxide. These are species which are able to obtain energyfrom the oxidation of hydrogen and concomittantly synthesize cellmaterial by the reductive assimilation of carbon dioxide via theribulose bisphosphate cycle as used by plants to produce biomass.

The preferred biocatalysts of this invention are prepared by fixing aselected microorganism to a porous solid surface, preferably a porousparticulate solid carrier, or support. Higher culture densities can beobtained by immobilization of the microorganisms than, e.g., suspendedcell systems, and high flow rates are feasible since washout cannotoccur when the microorganism is immobilized. The selection of thecarrier can affect the performance of a biocatalyst in the practice ofthis invention since pore dimensions and surface characteristics governboth the degree of biological colonization and transport of substrateand metabolic products. Biocatalysts useful in the practice of thisinvention, and methods for their preparation, both in terms of suitablemicroorganisms and carriers, and technique for immobilization andattachment of a selected microorganism to a support, are described,e.g., in U.S. Pat. Nos. 4,775,650 and 4,882,066, supra; and particularlyU.S. Pat. No. 4,859,594, supra; the disclosures of each of which isherewith incorporated and made of record. The use of a chitinoussubstrate deposited on a porous solid substrate, as described, e.g., inU.S. Pat. No. 4,775,650, makes the surface particularly favorable forbiological colonization. The following lists preferred microorganisms,and preferred porous solid substrates upon which the microorganisms canbe immobilized and attached, to form a select class of biocatalysts foruse in the practice of this invention, to wit:

                  TABLE                                                           ______________________________________                                                       Identifying                                                    Microorganism  Number     Substrate                                           ______________________________________                                        Xanthobacter autotrophicus                                                                   ATCC-43050 Diatomaceous earth                                  Pseudomonas fluorescens                                                                      ATCC-55360 Diatomaceous earth                                  Pseudomonas cepacia                                                                          ATCC-55362 Diatomaceous earth                                  Pseudomonas fluorescens                                                                      ATCC-55361 Diatomaceous earth                                  ______________________________________                                    

Strain ATCC-43050 is a publicly available strain. Samples of each of theother three strains listed above, ATCC-55360, ATCC-55362, andATCC-55361, were deposited with the patent depository of the AmericanType Culture Collection (ATCC), 12301 Parklawn Drive, Rockville, Md.20852 on Sep. 18, 1992, and were assigned the accession numbers statedabove. These deposits were made pursuant to contracts between ATCC andthe assignee of this patent application, Board of Supervisors ofLouisiana State University and Agricultural and Mechanical College. Thecontracts with ATCC provide for permanent availability of the progeny ofthese strains to the public on the issuance of the U.S. patentdescribing and identifying the deposits or the publication of the layingopen to the public of any U.S. or foreign patent application, whichevercomes first, and for availability of the progeny of these strains to onedetermined by the U.S. Commissioner of Patents and Trademarks to beentitled thereto according to 35 U.S.C. § 122 and the Commissioner'srules pursuant thereto (including 37 C.F.R. §§ 1.14 and 1.801 et seq.,with particular reference to 886 OG 638). The assignee of thisapplication has agreed that if any of the strains on deposit should dieor be lost or destroyed when cultivated under suitable conditions, itwill be promptly replaced on notification with a viable culture of thesame strain. These porous solid substrates, as indicated, provide asurface on which whole cells can be attached in forming a biocatalyst.Suitable solid surface substrates to which these, and othermicroorganisms can be attached to form biocatalysts are described e.g.,in U.S. Pat. No. 4,882,066 at Column 3, lines 12-34, herewithincorporated by reference and made part of this disclosure. Thebiocatalyst is packed into the reactor and used in a steady stateoperation. Since the population remains in place within the reactor, astable culture is continuously contacted with a sterile inlet feed.There is no necessity of separating a fluid from a solid phase, sincethe catalyst bed is fixed in place; and there is minimal backmixing. Therates of flow of the air (oxygen) and the conditioned liquid are set tomaintain the dissolved oxygen (DO) concentration above some minimumnecessary to the function of the selected microorganism up to a level atwhich the selected microorganism can function to decompose the toxicsubstance in the liquid influent. Preferably the flow rates are set at alevel to supply the amount of oxygen which is optimum to the function ofthe selected microorganism; which for some microorganisms may be thepoint of saturation of the conditioned liquid influent with oxygen. Ingeneral, the concentration of oxygen in the conditioned liquid influentranges from about 0.1 mg/L to about saturation, preferably from about 5mg/L to about 6 mg/L, dependent upon the microorganism. The reactoroperating pressure is controlled by regulating the pressure, P, at whichthe gas is delivered to the reactor. Oxygen concentration is increasedby raising the air delivery pressure, and can also be increased byadmixing pure oxygen in the gas supply. Overpressure can be controlledby relief valves on the influent line or reactor, or both. The flow rateof the contaminated liquid influent, or feed water, and recycle flow tothe reactor, can be controlled by the pumping rates.

The following is illustrative, and exemplifies the best mode ofoperating a preferred type of microbubble generator and reactor, asemployed in the practice of this invention. The microbubble generatorand reactor employed are those described by reference to FIGS. 1-2.

EXAMPLE

The microbubble chamber of the microbubble generator was first packedwith spherical glass beads of 0.5 to 3 mm diameter.

Air and an aqueous influent were then introduced into the microbubblechamber of the microbubble generator, the first mixing between the gasand liquid phases occurring with the creation of bubbles at the locationof convergence between the gas and liquid phases at the gas and liquidinlets. Under an applied pressure, mixing between the gas and liquidphases was progressively increased as the fluids were transportedthrough the microbubble chamber, the number of the bubbles increasingand the size of the bubbles decreasing. On entry, and passage of themixture through the annular inlet into the venturi tube, the oxygenatedinfluent was forced through the restricted opening the cross-sectionaldiameter of which ranged from 1.2 mm to 0.8 mm, the number of bubbleswere further increase and their diameter further decreased, theoxygenated influent on egress therefrom visibly resembling a liquidwithin which was dispersed a fine mist. The oxygenated effluent wheninput to a reactor was found very well suited to providing a continualoxygen transfer to a biocatalyst constituted of a microbial massattached to a solid support, or for facilitating export from the reactorof metabolic end products.

FIG. 3 graphically depicts the effectiveness of the microbubblegenerator in saturating an aqueous stream at flow rates between about 50milliliters per hour, ml/hr, to 550 ml/hr, with oxygen at various airflow rate, viz., 75 ml/min, 130 ml/min, 200 ml/min and 260 ml/min,respectively. The direct oxygen (D.O.), or amount of oxygen inmilligrams per liter, mg/L absorbed from the air by the water at thesedifferent flow rates is given on the y-axis, and the liquid flow rate isin ml/hr through the microbubble generator is given on the x-axis.

It is thus apparent that liquid stream deficit in oxygen can be readily,effectively oxygenated by contact with an oxygen-containing gas inapparatus as described. Hence liquid waste streams, where oxygen ispresent in limited amounts, if at al, can be readily oxygenated by theapparatus of this invention.

It is apparent that various modifications and changes can be madewithout departing the spirit and scope of this invention.

Having described the invention, what is claimed is:
 1. An apparatus forfacilitating high oxygen levels in a liquid admixed with anoxygen-containing gas suitable for contact and reaction with microbialinocula, comprising:(a) a microbubble generator comprising:(i) a firstchamber packed with inert solid particles of small diameter, said firstchamber having at least one inlet through which an oxygen-containing gasand a liquid can be admitted under pressure into said first chamber andcontacted together to form mixed liquid-gas phases in which the gas isdispersed as bubbles, and an outlet through which the mixed liquid-gasphases can be removed from said first chamber; and (ii) a second chamberof venturi configuration, said second chamber having an inlet connectedto the outlet of said first chamber through which the mixed liquid-gasphases from said first chamber can be admitted, and forced therethroughunder pressure to further reduce the size of the bubbles of the mixedliquid-gas phases, and an outlet for the discharge of the mixedliquid-gas phases; and (b) a reactor adapted to hold microbial inocula,said reactor being connected to the outlet of said second chamberthrough an inlet into which a stream of the liquid-gas phases from themicrobubble generator may be introduced for reaction.
 2. The apparatusof claim 1 wherein the microbubble generator is of elongate shape havingfirst and second ends, wherein each inlet of said first chamber islocated near the first end, and wherein the second chamber is of aventuri configuration the inlet of which is located near the second endand the outlet of which is located near the first end.
 3. The apparatusof claim 2 wherein a drain outlet is located near the second end.
 4. Theapparatus of claim 1 wherein an enclosing wall forms the first chamber,said enclosing wall having a cover which, on removal, provides a meansfor the addition or removal from said first chamber of said inert solidparticles.
 5. The apparatus of claim 1 wherein said first chamber has atleast one inlet for a gas and at least one inlet for a liquid, said gasand liquid inlets being concentric with respect to one another.
 6. Theapparatus of claim 1 wherein baffles are mounted in the first chamber.7. The apparatus of claim 1 wherein baffles are mounted in the secondchamber.
 8. The apparatus of claim 1 wherein baffles are mounted in boththe first and second chambers.